An interconnect circuit board of an LTCC design is a physical realization of electronic circuits or subsystems made from a number of extremely small circuit elements that are electrically and mechanically interconnected. It is frequently desirable to combine these diverse electronic components in an arrangement so that they can be physically isolated and mounted adjacent to one another in a single compact package and electrically connected to each other and/or to common connections extending from the package.
Complex electronic circuits generally require that the circuit be constructed of several layers of conductors separated by insulating dielectric layers. The conductive layers are interconnected between levels by electrically conductive pathways, called vias, through a dielectric layer. The conductors useful in LTCC technology are typically thick film conductors. LTCC multilayer structures allow a circuit to be more compact than traditional Al2O3 substrates by allowing vertical integration.
Similar to other thick film materials, thick film conductors are comprised of an active (conductive) metal and inorganic binders, both of which are in finely divided form and are dispersed in an organic vehicle. The conductive phase is ordinarily gold, palladium, silver, platinum or alloy thereof, the choice of which depends upon the particular performance characteristics which are sought, e.g., resistivity, solderability, solder leach resistance, bondability, adhesion, migration resistance and the like. In multilayer LTCC devices, additional performance characteristics are sought for internal conductor lines and via conductors which include minimization of conductor line “sinking” into the top and bottom dielectric layers on firing, minimal resistivity variation on repeated firing, optimal interface connectivity of line conductor to that of via-fill conductor, and optimal interface bonding of via-fill conductor to that of surrounding ceramic materials.
LTCC thick film compositions and tapes have been used in prior art high frequency applications, such as telecommunications, automotive or military applications including radar, for its multilayer, cofiring and flexible design capabilities. Many properties are required of the conductors used in the fabrication of multilayer circuits for high frequency including microwave applications, including desired resistivity, solderability, solder leach resistance, wire bondability, adhesion, migration resistance, and long term stability.
In addition to the proper level of conductivity and other properties listed above, there are many secondary properties which must also be present such as, wire bondability, good adhesion to both ceramic and thick films, solderability and compatibility to other thick films, both surface and buried, long-term stability without little or less properties degradation.
As would be expected, a most critical variable in the technology of thick film conductors for use in LTCC devices is the resistivity variation by interaction with surrounding ceramics. Of particular importance in this regard has been the incorporation of high-melting refractory glasses and glasses have little or no miscibility with the remnant glasses present in the surrounding ceramics. Furthermore, additional incorporation of metal oxide and non-metal oxide binder materials in the composition increases the densification of the conductor composites and/or control the growth of crystalline materials into the conductor composites which would change the resistivity of the composites, a non-desirable result.
U.S. Pat. No. 5,744,232 to Bailey discloses a thick film metallization compatible with LTCC that display very low microwave insertion losses commensurate with those of thin film gold. The electrical performance of the metallization is attained by using a spherical metal particle shape and uniform particle size distribution in the thick film paste.
Another issue of concern with prior art via-fill compositions involves the entrapment of water in the via composition which upon processing (firing) leads to an increase in the vapor pressure of the water and “explosion” of the LTCC structure.
The present invention overcomes problems associated with prior art thick film conductor compositions for use in LTCC and microwave applications. In particular, the present invention provides thick film compositions and LTCC structures, which provide superior refire stability.